Bouncing on an asteroid, in search of the solar system's roots

NASA hopes spacecraft's asteroid mission will open 'time capsule' of life

By Kenneth F. Chang | New York Times

September 6, 2016

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Bouncing on an asteroid, in search of the solar system's roots

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For the next two years, NASA's latest robotic spacecraft will be chasing down an asteroid near Earth in the hopes of scooping up some of the most primordial bits of the solar system.

The premise of the mission for the spacecraft, OSIRIS-REx, is simple: Fly to an asteroid, grab some of the rock and bring it back to Earth, where scientists will study some of the pristine ingredients that went into the making of the solar system, including possibly the building blocks of life.

"What was that beginning organic material like?" James L. Green, the director of NASA's planetary science division, said in an interview. "That's what's really exciting about this. This is what we want."

The details are a bit more complicated.

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The spacecraft is sitting on top of an Atlas 5 rocket at Cape Canaveral, Florida, ready for launching on Thursday on a seven-year mission.

Once off the ground, OSIRIS-REx - a shortening of Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer - will be aiming to get close to the asteroid Bennu.

"It's 500 meters or so in size, about the height of the Empire State Building," Green said.

Discovered in 1999, Bennu is a carbon-rich, almost black asteroid. Data from NASA's Spitzer Space Telescope and radar measurements by ground-based radio telescopes suggest it is a "rubble pile" with pebbles about half an inch wide on the surface.

Scientists believe that it is a conglomeration of leftovers, largely unchanged during the last 4.5 billion years.

"It's a time capsule from the earliest stages of solar system formation," said Dante Lauretta, a professor of planetary science and cosmochemistry at the University of Arizona who is the mission's principal investigator.

OSIRIS-REx will survey Bennu for more than a year to select the site where it will grab the sample of rock. In July 2020, the spacecraft, about the size of a sport utility vehicle, is to slowly descend and bounce off the surface like a pogo stick at a gentle pace of a quarter-mile per hour. A sampling head, which looks like an automobile air filter, will shoot a burst of nitrogen to kick up dirt and small rocks during the three to five seconds it is in contact with the surface.

The goal is to collect at least a couple of ounces of material and possibly as much as 4.4 pounds. The spacecraft carries enough nitrogen to attempt to extract material three times if necessary.

After departing Bennu in 2021, OSIRIS-REx will pass by Earth in September 2023, dropping off a capsule with the samples that will land via parachute in a Utah desert.

A Japanese mission, Hayabusa 2, will similarly collect samples from another carbon-rich asteroid, but the OSIRIS-REx scientists view the missions as complementary, not redundant.

Lauretta said he was particularly interested in gleaning information about organic molecules like amino acids, the building blocks of proteins that are known to float in outer space. One question is whether Bennu contains higher concentrations of the 20 amino acids used by forms of life on Earth compared with dozens of others not found in living organisms.

Studying this asteroid could also come in handy if it is ever on a collision course with Earth. With about a 14-month orbit, Bennu passes fairly close once every six years.

In 2035, it will make a particularly close approach, passing within the orbit of the moon, and Earth will give enough of a gravitational kick that astronomers cannot precisely predict where the asteroid will go after that.

There is a small chance - 1 in 2,700 - that Bennu will smack into Earth sometime from 2175 to 2196.

Bennu is not large enough to wreak planetwide extinctions - the asteroid that is thought to have killed off the dinosaurs 66 million years ago was about 6 miles wide - but a collision would be devastating. Impact would occur at more than 27,000 mph, unleashing energy equivalent to 1,450 million tons of TNT and carving a crater almost 3 miles wide and 1,500 feet deep, Lauretta calculated.

One of the uncertainties in predicting Bennu's path is that gravity is not the only force acting on it. As the asteroid rotates, the dark surface absorbs sunlight and then radiates heat. The radiated heat pushes the asteroid like a rocket thruster, a small effect that becomes significant over time.

With detailed observations of the surface, scientists will be able to test their models of this effect with the actual orbits of Bennu.

For Lauretta, OSIRIS-REx's launching is a culmination of a dozen years of work. In 2004, officials at Lockheed Martin discussed the idea of an asteroid return mission with Michael J. Drake, the head of the Lunar and Planetary Institute at Arizona. Drake invited Lauretta to take part.

Lauretta came up with the name. "That's entirely my fault," he said.

Meanwhile, engineers at Lockheed Martin had to figure out how to grab a bit of asteroid. The company held a contest. Many of the proposals suggested landing and scooping, but an engineer named James Harris came up with the pogo stick idea, testing out a prototype in his dirt driveway with a plastic cup and an air compressor.

"Now instead of worrying about how to anchor a spacecraft to an asteroid," said Richard Kuhns, the program manager for OSIRIS-REx at Lockheed Martin Space Systems, "in almost no gravity, you get to gently touch it, take your sample and then back away. Much, much safer for a spacecraft in that environment."